The reason multitasking sucks is because it fries our ability to filter out irrelevant information. Being perpetually underslept similarly sucks because it too reduces out ability to filter out the irrelevant stimuli, as Michael Chee, a sleep researcher at Duke-National University of Singapore, explained to us.

In one of his labs experiments, he asked subjects to take part in a dumb task: hitting a button when a light flashes. But, as he explains, if you're underslept, your vigilance drops and you will start to miss responses—even for something as simple as hitting a button.

"The danger of this is when doing something like driving, where you tune out at possibly the wrong time. It also relates to people who have to monitor for information: if information is coming from off screen, you might miss it because your brain tuned out—and tune outs are far more frequent (when you're sleep deprived)," he says. "If you are driving down the road, you want to focus on what is directly ahead of you, but if a child comes by the side of the road, you do not want to miss that."

Research suggests we need 8.1 hours of sleep a night. Americans get six hours a night on average, though we spend 7.5 in bed.

So how much do you need? While Chee's lab uses folks with less than six hours of sleep as their threshold for being sleep deprived, he tells us that you should take an experimental approach to your sleep hygiene: test how many hours of sleep at which lengths of time work for you—then stick to that.

This is why sleep aficionados say that you need to have the longest uninterrupted sleep as possible: because if you hit the snooze button and doze off again, you don't get back into the crucial REM stage right away. Meaning that you're time spent dozing won't be nearly as beneficial as if you were properly sleeping.

It gets complicated. Some research suggests that sleep debt—the idea of "catching up" on sleep over the weekend—doesn't really work. But Russell Foster, an Oxford neuroscientist, says that addressing sleep debt is a part of balancing homeostatic drive.

"If you disturb a fish or a fruit fly or whatever during their rest period, it will need more sleep the following night," Foster tells the Telegraph. "And if it doesn’t get it, the organism will seek more of it. Like starving something of food."